Stealth

One of the great advantages to the lensatic compass is their use in night time navigation. The compass bezel is illuminated with a radioactive isotope. All modern lensatic compasses use Tritium, older compasses used to use Radium, but their use was linked to cancer, so don't use them.

The radioactive tritium particles are low energy beta particles and are not harful for external use, (they cannot pass through clothing or skin). As long as you don't swallow them you are fine.

The reason for the illumination is so that the compasses can be used without any external light source. The military, especially special forces, love this. It allows for "dark" navigation.

Other information

All radioactive isotopes have a half life. And Tritium's is 12 years, so in 12 years your compass will be half as bright as it is the day you bought it, then 12 years later, another half life. And so on.